An Etiologic and Early Marker Study Program is carried out within an NCI study of selected screening procedures for the early detection of prostate, lung, colon, and ovarian cancer (The PLCO Trial). The Trial will include 74,000 screening arm participants (37,000 men, 37,000 women; age 55-74) and an equal number of non-screened controls. For etiologic and early marker investigations, questionnaire data and biologic samples are collected. Etiologic and early marker studies for specific cancers and related conditions will be phased in as sufficient numbers of cases accrue. Currently, two major efforts are underway to evaluate genetic and environmental (including lifestyle-related) determinants of adenomatous polyps and prostate cancer. Other studies include risk factors for BPH and early detection biomarkers for prostate cancer.